The present invention relates to the field of signal transmission, and more particularly to a bias tee for inserting low frequency signals on high frequency transmission lines.
Bias tees are widely used to insert low frequency (e.g., DC) signals onto high frequency (e.g., radio frequency (RF)) lines. FIG. 1 shows a schematic diagram of a conventional bias tee 100. The bias tee 100 includes a high frequency input terminal 110 and a low frequency input terminal 120. The bias tee 100 also includes a first resistor (R1) 135 coupled between the high frequency input terminal 110 and ground, and a series-connected capacitor (C1) 140 and second resistor (R2) 150 coupled between the high frequency input terminal 110 and ground.
In operation, a high frequency (e.g., RF) transmission line (not shown) is coupled to the high frequency input terminal 110 and a low frequency (e.g., DC) signal (not shown) is applied at the low frequency input terminal 120. It will be noted by those skilled in the art that the high frequency signal (not shown) present on the high frequency transmission line is grounded through first resistor 135. Additionally, the high frequency signal is prevented from reaching the low frequency input terminal 120 by the first capacitor 140 and the second resistor 150. As is well known in the art, the first capacitor 140 will operate as a virtual short circuit with respect to high frequency signals, thereby shunting all such signals to ground through the second resistor 150. The low frequency signal applied at low frequency input terminal 120 however will pass through to the high frequency transmission line coupled to the high frequency input terminal 110 (since first capacitor 140 operates as an open circuit with respect to the low frequency signal). In this manner, a DC bias signal may be carried on the high frequency transmission line along with the high frequency signal.
However, in the bias tee 100 shown in FIG. 1, a high frequency signal which is present at high frequency input terminal 110 must pass through both the capacitor 140 and the second resistor 150 before reaching ground which increases the total impedance to ground. Furthermore, when the bias tee is formed monolithically, the high frequency signal must pass through a via between the first capacitor 140 and ground formed on a lower surface of the monolithic substrate (e.g., glass). Since the diameter of the via is typically small, the via exhibits a high inductance, and therefore limits the bandwidth of the bias tee 100. It will be noted by those of ordinary skill in the art that a wide bandwidth is necessary for almost all high frequency applications.
Therefore, there is presently a need for a wide bandwidth bias tee.
The present invention includes a wide bandwidth bias tee including a high frequency terminal, a first resistor coupled between the high frequency terminal and a capacitor, a second resistor coupled to the capacitor, the second resistor coupled in series with the first resistor, and a low frequency terminal, the low frequency terminal coupled to the second resistor.
The present invention also includes an integrated circuit bias tee comprising a substrate including a grounding pedestal layer formed therein, a capacitor plate structure disposed on the grounding pedestal layer, at least one first resistor formed on a first side of the capacitor plate structure and coupled thereto, and at least one second resistor formed on a second opposing side of the capacitor plate structure and coupled thereto.
Additionally, the present invention comprises a method for increasing the bandwidth of a bias tee including, disposing a capacitor of the bias tee on a first surface of a monolithic substrate and disposing a ground pedestal in the monolithic substrate and coupling a plate of the capacitor directly thereto.